Unit 3: Structural Organization of Flower
Table of Contents
Syllabus Reference: Structure and Functions of flower, Structure of anther and pollen; Structure and types of ovules; [cite_start]Types of embryo sacs, organization and Ultrastructure of mature embryo sac [cite: 554-555].
1. Flower Structure
A flower is a modified shoot meant for sexual reproduction.
- Essential Whorls: Androecium (Stamens, Male) and Gynoecium (Carpels, Female).
- Non-Essential Whorls: Calyx (Sepals) and Corolla (Petals). Protection and attraction.
2. Anther and Pollen
Structure of Anther: Typically bilobed and dithecous (4 microsporangia).
Wall Layers (Outer to Inner):
- Epidermis: Protection.
- Endothecium: Fibrous bands (alpha-cellulose) help in dehiscence (release of pollen).
- Middle Layers: Ephemeral (short-lived).
- Tapetum: Innermost nutritive layer. Nourishes developing pollen. Secretes sporopollenin (exine) and pollenkitt.
Structure of Pollen Grain (Microspore):
- Exine: Outer, hard layer made of Sporopollenin (most resistant organic material). Has germ pores.
- Intine: Inner, thin layer made of Pecto-cellulose.
- Cell State: Shed at 2-celled stage (Vegetative cell + Generative cell) or 3-celled stage.
3. Ovule Types
The Megasporangium is called the Ovule.
- Orthotropous (Atropous): Upright. Micropyle, Chalaza, and Funiculus lie in a straight line (e.g., Polygonum).
- Anatropous (Inverted): Body inverted 180°. Micropyle lies close to funiculus. Most common type (82% angiosperms).
- Campylotropous: Body curved, micropyle bends down.
- Amphitropous: Both body and embryo sac curved (Horse-shoe shaped).
4. Embryo Sac (Female Gametophyte)
Polygonum Type (Monosporic, 7-celled, 8-nucleate): The most common type.
Structure of Mature Embryo Sac:
- Egg Apparatus (Micropylar end): 1 Egg cell + 2 Synergids (help in guiding pollen tube via Filiform Apparatus).
- Central Cell: Large, contains 2 Polar Nuclei (fuse to form Secondary nucleus, 2n).
- Antipodals (Chalazal end): 3 cells (usually nutritive/degenerative).